Using the proper equipment, any high-definition (HD) source can be connected to more than one display, at least in theory. The ease with which this can be accomplished varies though. There are five different standards for connecting a display to a source. Equipment for duplicating a signal varies with each display standard, but all work on the same basic concept of duplicating a video signal.
Display Standards
There are five display standards that support HD video: HDMI (High-Definition Multimedia Interface), VGA (Video Graphics Array), DVI (Digital Video Interface), component video and DisplayPort. DisplayPort can be safely ignored, as it is not generally used for television. DVI can also be ignored, as it is wire-compatible with HDMI; the two are completely interchangeable with the proper adapter. While most HDTVs being sold as of 2009 include VGA inputs, most HD sources do not have VGA outputs, and thus VGA can also be ignored. That leaves only HDMI and component.
Distribution
Both HDMI and component are display standards for which "distribution amplifiers" are commonly available. A distribution amplifier is a device that splits a signal into two (or more) and amplifies each signal so that it has sufficient power to drive a display. Technically, the amplification only exists for component video, which is analog, as a digital signal needs only to be copied. However, the name has stuck for the HDMI equivalent of this type of device.
These devices, while not common in retail, are readily available from audio/video suppliers. They do carry a significant price tag. The cheapest HDMI video distributor at the time of this writing was in the $250 range. At that price, it's only going to be worth the cost in a high-end home theater or a store display setup. However, if two or more duplicated digital displays are what is necessary, there really isn't much choice in the matter. On the other hand, component video distributors are quite a bit cheaper than their HDMI equivalent. These can be found for around $80.
Alternatives
For the home theater, at least, it's probably more cost-effective to simply acquire a second source device, whether it's an HD DVR or a Blu-ray player. Not only is cost a valid consideration, but copyright protection can rear its ugly head when dealing with HDMI. HDCP (High-bandwidth Digital Content Protection) is not always supported by distributor devices, and with some HDCP-protected content, their use may be prevented by the publisher. At least with a second source device, everything will work without any problems.
Tags: component video, also ignored, HDMI component, HDMI equivalent, home theater, second source
